- ZA = Zethilius Associates
- 30 = caliber
- 5.8 = length in calibers (5.8x.30=1.74")
- W = bullet type (in this case tungsten core)

I am omitting the weight in the nomenclature, because when these are released there will be only two types in a given projectile length;

1 - The "M" will be a target solid, optimized for form factor.

2 - The "W" is a frangible optimized for both form factor, and mass. This is ideal for ELR, or for hunting. The shortest "W" projectile in each caliber series will be dimensioned for magazines, and twist rate compatibility with readily available, off-the-shelf, barrels. In the ZA30/5.8-W example, the 8.3" twist requirement is available from Hart (8.0"), Shilen (8.0"), Lilja (8.0"), and Pac-Nor (8.0" & 7.0"). If you want a cut-rifled barrel, Bartlein, and a few others will produce it on a special order. The 27 caliber twist will work for all 6 caliber, or shorter, "W" projectiles.

For ZA projectiles in excess of 6.0 calibers, a gain twist in a linear, exponential, or acceleration compensated, geometry is ideal... but they will spin the jackets off of conventional bullets, so you are committed to a dedicated system once this line is crossed.

If I have totally confused you, feel free to ask me about a specific bullet.

Mark,

Both the 175, and 240, should stabilize in the 8" twist, but obviously the 175 SG will be lower. A higher MV will add stability to this projectile.

P.S.,
I did not anticipate going into great detail in this thread on the ZA projectile specifications, and am not holding back information without cause. I will post photographs of these at sometime prior to the meet.

Last edited by noel carlson; 02-05-2010 at 10:49 PM.

I'm just curious as to the length in inches of these projectiles and the min. twist rate in inches needed to stabilized them at sea level.

Thanks,

Jon

__________________
In the beginning God created the heavens and the earth. Genesis 1:1

"And can the liberties of a nation be thought secure when we have removed their only firm basis, a conviction in the minds of the people that these liberties are the gift of God?" Thomas Jefferson - Notes on the State of Virginia

I'm just curious as to the length in inches of these projectiles and the min. twist rate in inches needed to stabilized them at sea level.

Thanks,

Jon

Jon,

If you take the caliber length of the bullet, i.e. 5.8, and multiply it by the bore caliber, .308 you will get the bullet lenght in inches, in this case 1.78". For comparison the 210 Berger is about 1.48" or 4.8 calibers. The 265 HAT is 1.785" or 5.28 calibers.

Noel can correct me if I'm wrong, but I'm guessing the twist rate is bsed on the diameter to length ratio of the bullets (measured in calibers). The Twist for the 5.8 caliber bullets being 27 clibers (or 8.3" in a .308) and for the 6.5 caliber bullets being 18 calibers (or 5.5" in a .308) Not sure if that remains the same when going to a different bore size?

If you take the caliber length of the bullet, i.e. 5.8, and multiply it by the bore caliber, .308 you will get the bullet lenght in inches, in this case 1.78". For comparison the 210 Berger is about 1.48" or 4.8 calibers. The 265 HAT is 1.785" or 5.28 calibers.

Noel can correct me if I'm wrong, but I'm guessing the twist rate is bsed on the diameter to length ratio of the bullets (measured in calibers). The Twist for the 5.8 caliber bullets being 27 clibers (or 8.3" in a .308) and for the 6.5 caliber bullets being 18 calibers (or 5.5" in a .308) Not sure if that remains the same when going to a different bore size?

-Mark

4,08

Hi Mark,

I know the caliber and length of bullet thing, but how did you arrive at the min twist numbers? I must've have missed that otherwise, but have not combed through every post on this thread with a fine tooth comb. My bad, if I missed that.

More than that, I just think it'd be easier for most of us if we saw these numbers in units we're more familiar with, namely inches. That's the language most of us have used for years hear and so it gives a quick and easy reference. Obviously differnent 'languages' and that's fine, I just think it'd be easier for potential customers to understand what they are dealing with potentially if someone converted that list of bullets from Noel to inches.

Thanks,

Jon

__________________
In the beginning God created the heavens and the earth. Genesis 1:1

"And can the liberties of a nation be thought secure when we have removed their only firm basis, a conviction in the minds of the people that these liberties are the gift of God?" Thomas Jefferson - Notes on the State of Virginia

Gyroscopic stability can be determined mathematically. Bryan Litz has publicized the formula, based on projectile mass, and length, in Precision Shooting Magazine I believe. This sets a minimum parameter for dynamic stability. To really nail down the actual twist requirement, nothing substitutes for empirical testing.

- Noel

Last edited by noel carlson; 02-06-2010 at 01:13 AM.

Just to provide some idea of the non-equivalence of "gyroscopic", and "dynamic" stability; The ZA375/6.5-M is supposed to be stable in a 1: 10" twist based on the calculator which you linked.

In reality, it requires a 1: 6.5" exit twist at 3,000 fps. Some very serious errors have been made by projectile manufacturers who relied too heavily on mathematical models. Bullet flight stability is very complex.